Conventional ground drilling equipments or techniques normally drill holes with a single or outer diameter (OD) target. Depending on the ground condition, a casing sometimes is inserted in the ground to prevent collapse of soil when the ground condition is loose. In the case where the ground strata are stable, no casing is necessary to be inserted to the ground. In either case, all the materials inside the hole will needed to be excavated away in the process of drilling.
The single DTH percussion hammer is well known in its use for general ground drilling. The DTH percussion hammer can be driven by either compressed air or pressurized fluid such as water.
A cluster drill of DTH percussion hammers is an implementation of a plurality of DTH hammers arranged and allocated in a cylindrical housing of which the OD defines the diameter of the hole drilled. One such implementation is shown in FIG. 1. The plurality of DTH hammers can also be arranged and allocated in an annulus housing having an OD and an inner diameter (ID) designed to drill ring holes with a particular size. One such implementation is shown in FIG. 2. In both cases, the OD of the hole to be drilled can range from a minimum of 300 mm to any diameter length. There is no theoretical maximum limit to the size of the hole as there can be many different possible arrangements of the plurality of DTH hammers. In the case of drilling annulus ring holes, the ID of annulus ring hole can range from a minimum of 200 mm to any larger diameter length.
Contemporary designs and arrangements for the cluster drill of DTH percussion hammers often have centralized supply sources of compressed air or pressurized fluid for their hammer driving mechanisms. The compressed air or pressurized fluid delivery paths branch out to all the DTH percussion hammers so that each of them can actuate its corresponding piston to strike on its front drill bit. With this configuration, the cluster drill of DTH percussion hammers assembly can hammer and penetrate homogenous hard materials or ground formation over its drilling area. In other words, when all the DTH percussion hammers are simultaneously impacting the homogenous hard materials or ground formation, there will be enough evenly distributed reaction force feedback on to the drill bits and in turn pushing back on to the pistons to facilitate the hammering cycle.
However, a problem arises in practice when the cluster drill of DTH percussion hammers is needed to drill mixed ground formation comprising materials of different rigidities. While the hard ground formation can provide enough reaction force feedback on to a drill bit to facilitate the continuation of hammering cycle of its corresponding piston, soft ground formation does not provide sufficient reaction force, causing the drill bit to drop and rest on the drill bit retaining ring. Following the drill bit, the piston also rests on top of it. The compressed air or pressurized fluid then escapes directly from the drill bit through the main exhaust holes in piston instead of going through the bottom chamber, which normally feeds the piston for the return stroke in a normal hammering cycle if the ground is hard enough to provide sufficient reaction force. At this point, the DTH hammer is at the maximum flushing position with the compressed air or pressurized fluid supplied to it being directly released out through its bottom of the drill bit. This condition is called the “direct exhaust phenomenon.”
The condition described above is the result of that compressed air or pressurized fluid being delivered from a single centralized supply source for the operation of all the DTH percussion hammers in the cluster of DTH percussion hammers assembly. The direct exhaust phenomenon occurred in the DTH percussion hammers (or even in a single DTH percussion hammer) that are impacting soft ground. It led to the bypassing of all compressed air or pressurized fluid from the centralized supply source through these direct exhaust path(s) because of the much less flow resistance through the bottom of the drill bit(s); as opposed to the much higher flow resistance experienced when the compressed air or pressurized fluid is driven on the piston(s) of those DTH percussion hammer(s) that are impacting on hard ground.
Furthermore, due to the release of the compressed air or pressurized fluid through the lesser flow-resistive path(s) associated with the DTH percussion hammer(s) that are impacting soft ground; there is insufficient compressed air or pressurized fluid left to drive the other piston(s) of those DTH percussive hammer(s) that are impacting hard ground. Subsequently, the piston(s) of the DTH percussive hammer(s) that are impacting hard ground cease. Therefore, when the cluster drill of DTH percussion hammers encounter mixed ground formation during drilling, it cannot penetrate any more at that ground depth and the drilling cannot proceed further.
In fact, the aforementioned condition is the limitation of application of contemporary designs of cluster drills of DTH percussion hammers in drilling works, that is they can only be used in drilling homogeneous or competent rock strata, but not mixed ground formation. Therefore, there is a need for equipments and/or methodologies for controlling the operation of cluster drill of DTH percussion hammers for drilling mixed ground formation.